Method and device for cryogenic cooling

ABSTRACT

Method for cryogenic cooling of a first fluid by heat exchange with at least one second fluid in a heat exchanger, the first fluid and/or the second fluid being at a temperature between −100° C. and −273° C., wherein the heat exchanger is of the type with polymer microtubes, i.e. comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 1 cm, one of the first and second fluids being circulated inside said microtubes while the other fluid is circulated around said microtubes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a § 371 of International PCT Application PCT/EP2021/057695, filed Mar. 25, 2021, which claims the benefit of FR2005565, filed May 27, 2020, both of which are herein incorporated by reference in their entireties.

FIELD OF THE INVENTION

The invention relates to a method and a device for cryogenic cooling.

The invention relates more particularly to a method of cryogenic cooling of a first fluid by heat exchange with at least one second fluid in a heat exchanger, the first fluid and/or the second fluid being at a temperature of between −100° C. and −273° C.

BACKGROUND OF THE INVENTION

The structure of cryogenic heat exchangers is usually bulky, costly and heavy. There are known exchangers made with plates or aluminum or metal tubes, for example. This type of exchanger is therefore poorly suited to certain applications where the weight or volume are critical (on board floating vessels or aircraft, for example). Other, less heavy, systems are known (such as shell and tube exchangers with polymer tubes), but they are not suitable for applications in cryogenic temperature ranges (below 100° C. for example), because these exchangers become brittle at these temperatures and cannot withstand the pressure differentials and/or encounter problems of tightness and performance.

One aim of the present invention is to overcome all or some of the aforementioned drawbacks of the prior art.

SUMMARY OF THE INVENTION

To this end, the device according to the invention, while conforming in other respects to the generic definition given in the above preamble, is essentially characterized in that the heat exchanger is of the type with polymer microtubes, i.e. comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 1 cm, one of the first and second fluids being circulated inside said microtubes while the other fluid is circulated around said microtubes.

Embodiments of the invention may also have one or more of the following characteristics:

-   -   the microtubes consist of at least one of the following         materials: polyether ether ketone (PEEK),         polytetrafluoroethylene (PTFE), polyetherimide, polyimide(s),         polyamide(s), polycarbonate(s) or any other plastic material         compatible with use at said low temperatures,     -   the microtubes consist of a material comprising a mixture of         polyetherimide (“Ultem”) and PEEK,     -   microtubes consist of a material having a mass per unit volume         of between 2700 kg/m³ and 900 kg/m³, and notably less than 1500         kg/m³     -   the diameter of the microtubes is between 0.1 mm and 5 mm,     -   pressure differential between the pressure of the fluid         circulated in the microtubes and the pressure of the fluid         circulated around the microtubes is between 1 bar and 100 bar,         and notably between 10 and 50 bar,     -   the heat exchanger comprises a casing in which the microtubes         are arranged, the casing comprising a first inlet communicating         with a first end of the microtubes, the casing comprising a         first outlet communicating with a second end of the microtubes,         the casing further comprising a second inlet and a second outlet         communicating with the space around the microtubes,     -   the microtubes are arranged in a bundle along a longitudinal         direction in the casing,     -   the bundles are arranged parallel to the longitudinal direction         and are preferably straight,     -   the microtubes are wound in coils, preferably distributed around         a central supporting core,     -   at least one of the two longitudinal ends of the microtube         bundle comprises a layer of rigid material such as a         thermosetting material, ensuring the cohesion of the microtube         bundle, the casing housing at least one elastic member stressed         in the longitudinal direction between a stop formed in the         casing and a longitudinal end of the microtube bundle, to ensure         the longitudinal retention of the microtube bundle while         allowing expansion or contraction relative to the casing in the         longitudinal direction,     -   the elastic member comprises at least one of a spring, a coil         spring, one or more spring washers, notably of the Belleville         type,     -   the casing comprises an elastic area lying along the         longitudinal direction, such as a bellows,     -   the method provides an exchange of heat between more than two         fluids, that is to say separate portions of the microtubes         and/or of the space around the microtubes receive separate flows         of fluid(s) for the purpose of exchanges of heat in the heat         exchanger,     -   the cooling takes place in a process of cryogenic refrigeration         and/or liquefaction of a fluid, the heat exchanger being located         in a cryogenic refrigeration and/or liquefaction device.

The invention also relates to a device for cryogenic cooling of at least one first fluid by heat exchange with at least one second fluid, comprising a heat exchanger providing heat exchange between the first fluid and the second fluid, the first and/or the second fluid being at a temperature between −100° C. and −273° C., the heat exchanger being of the type with polymer microtubes, that is to say comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 10 mm, the heat exchanger comprising inlets and outlets for the first and second fluids, providing circulation of at least one fluid inside said microtubes and circulation of the other fluid around said microtubes.

Additionally, according to possible characteristics: the exchanger comprises a working circuit containing a working fluid, the working circuit comprising at least one compressor for the working gas, at least one heat exchanger for cooling the compressed fluid, at least one expansion member for expanding the working fluid, at least one heat exchanger for heating the expanded working fluid, the at least one cooling heat exchanger and/or the at least one heating heat exchanger being of the type with polymer microtubes, that is to say comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 10 mm, and comprising inlets and outlets for a first flow of working fluid and another fluid having a separate temperature from the temperature of the first flow of working fluid, to provide a heat exchange between the first flow of working fluid and the other fluid.

The invention may also relate to any alternative device or method comprising any combination of the features above or below within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

[FIG. 1 ] shows a partial schematic view in longitudinal section illustrating a first example of the structure and operation of a cooling heat exchanger according to the invention,

[FIG. 2 ] shows a partial schematic view in longitudinal section illustrating a second example of the structure and operation of a cooling heat exchanger according to the invention,

[FIG. 3 ] shows a partial schematic view in longitudinal section illustrating a third example of the structure and operation of a cooling heat exchanger according to the invention,

[FIG. 4 ] shows an enlarged detail of the heat exchanger of [FIG. 3 ],

[FIG. 5 ] shows a sectional view of an example of an elastic member that may be used in such a heat exchanger,

[FIG. 6 ] shows a partial schematic view in longitudinal section illustrating a fourth example of the structure and operation of a cooling heat exchanger according to the invention,

[FIG. 7 ] shows a partial schematic view in longitudinal section illustrating an example of the structure and operation of a cryogenic cooling device that may use a cooling heat exchanger according to the invention,

[FIG. 8 ] shows a view in longitudinal section of another example of embodiment of such a cooling heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

The cryogenic cooling heat exchanger 1 shown schematically in FIG. 1 cools a first fluid 2 by heat exchange with a second fluid 3. For example, the first fluid 2 and/or the second fluid 3 is a temperature between −100° C. and −273 ° C. Although it works optimally at cryogenic temperatures, the invention may also be used at ambient temperature or between ambient temperature and cryogenic temperatures. The heat exchanger 1 is of the type with polymer microtubes, that is to say comprising a plurality of microtubes 4 made of polymer and having a diameter of between 0.1 mm and 1 cm. For example, the first fluid 2 is circulated inside said microtubes 4, while the second fluid 3 is circulated around said microtubes 4. The microtubes 4 are preferably non-porous.

The microtubes 4 preferably consist of at least one of the following materials: polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), polyetherimide, polyimides, polyamides, and polycarbonates, for example a mixture of 50% polyetherimide (“Ultem”) and 50% PEEK, and notably any appropriate material compatible with cryogenic temperatures. This PEEK/Ultem mixture may be extruded in the form of an amorphous material whose glass transition temperature (Tg) is around 180° C. This mixture, unlike the materials used in the literature, requires no post-cure. Furthermore, this mixture is essentially non-crystalline, and is reasonably strong and flexible as manufactured. Other crystalline materials mentioned in the literature would have relatively high coefficients of thermal expansion (CTE), in excess of 80 ppm/° C. in some cases.

Since the heat exchanger has to operate over a very wide temperature range, the construction materials must have coefficients of thermal expansion (CTE) that are strictly suitable. This is necessary in order to allow the various components of the device to expand and contract in unison over the operating temperature range. It is essential that the CTEs should be closely aligned in order to maintain the link between the material of the tube plate and the heat exchange tubes, as well as between the tube plate and the core of the structural support bundle.

All the structural components of the module are selected so as to have closely aligned or virtually identical CTEs.

Thus the coefficient of thermal expansion (CTE) of PEEK is 45, the coefficient of thermal expansion of Ultem is 45 and the coefficient of thermal expansion of epoxy resin is, for example, equal to 55.

This constituent material is compatible with cryogenic temperatures (up to several degrees Kelvin for example) and can withstand large pressure differentials (up to about 100 bar, for example).

The microtubes 4 preferably consist of a material having a mass per unit volume of between less than 2700 kg/m³ and notably less than 1500 kg/m³, for example within the range from 900 kg/m3 to 2700 kg/m3.

The thickness of the microtubes 4 is preferably between 0.01 mm and 1 mm, notably 0.05 mm. Additionally, the microtubes 4 preferably have a diameter of the order of 0.1 to several millimetres, notably one millimetre.

This allows a high level of heat exchange between the two fluids, while limiting the volume and weight of the heat exchanger. The mechanical resistance to the pressure differential between the parts of the exchanger subjected to high pressure and the parts subjected to a lower pressure is not affected at all.

The pressure differential between the pressure of the fluid 2 circulated in the microtubes 4 and the pressure of the fluid 3 circulated around the microtubes 4 may be between 1 bar and 100 bar, and notably between 10 and 50 bar.

The heat exchanger 1 may comprise a casing 5 in which the microtubes 4 are arranged. The casing 5 comprises a first inlet 6 communicating with a first end of the microtubes 4 and a first outlet 7 communicating with a second end of the microtubes 4. The casing 5 also comprises a second inlet 8 and a second outlet 9 communicating with the space around the microtubes. These two inlet/outlet pairs define two independent circuits for two fluids.

As illustrated, the microtubes 4 may be arranged in a bundle, for example parallel to a longitudinal direction in the casing 5 (and notably straight or substantially straight), or in any other geometric configuration.

For example, the microtubes 4 may be wound in coils and, for example, distributed in an organized manner around a central supporting core. This coil winding may serve not only to control the packing density, but also to provide a single mechanism for counteracting the potential dimensional shrinkage of the tubes at low temperature. In devices in which the microtubes are arranged in parallel, the shrinkage of the tubes due to thermal contraction may potentially exert a stress on the microtubes 4 that will be transmitted to the tube plate. This stress may lead to a fracture of the link between the material of the tube plate and the individual tubes, or, in extreme cases, a failure of the tube plate or the manifold itself.

The microtubes 4 wound into a coil make it possible to relieve the shrinkage stress by modifying their angle of winding in the device. The microtubes 4 are therefore not subjected to axial tension in the course of their shrinkage.

[FIG. 8 ] shows a view in longitudinal section of a possible example of embodiment of such a heat exchanger with microtubes 4 wound in a coil around a central core 20. The bundle of wound microtubes 4 may thus form a tubular entity whose two ends may be mounted, respectively, on the axes of two inserts 21, 22 mounted at the ends of the core 20.

The peripheral surface (which may be cylindrical, for example) of the bundle of microtubes 4 may be covered with a protective and/or supporting winding or layer 23.

The fluid 2 circulated in the microtubes 4 may be admitted, for example, transversely to an inlet at one longitudinal end of the exchanger and may flow out at the other longitudinal end, for example via passages 24 opening transversely to the core 20, and pass out via a central passage in an insert 22. The coolant, for its part, may travel longitudinally around the microtubes 4 in an opposite direction to the longitudinal progression of the first fluid 2.

Additionally, one or preferably both of the longitudinal ends of the bundle of microtubes 4 comprises a layer 13 of rigid material such as a thermosetting material (epoxy resin or other) that ensures the cohesion of the bundle of microtubes and can withstand cryogenic 0 temperatures. This rigid area may, notably, be used to ensure tightness between the two fluid circuits (for example, by means of one or more gaskets 18, notably 0-rings interposed between the casing 5 and the resin layer 13 (as illustrated in [FIG. 3 ]).

This body or layer 13 of solid resin material bonds the microtubes 4 at their ends, in order to prevent the fluid at high pressure from communicating with the fluid at low pressure when the 5 module is in operation. The resin used for this part bonds reliably with the constituent material of the microtubes 4, and also has a high glass transition temperature Tg (of the order of 150° C., for example).

This adhesion between the resin and the microtubes 4 is improved by the use of the aforesaid materials constituting the microtubes 4. This is because PEEK is a crystalline material whose surface is relatively “slippery” because of its low coefficient of friction. Therefore it is usually difficult to bond it to an adhesive resin. The incorporation of polyetherimide (Ultem) or equivalent, as stated above, in the composition of the microtubes 4 creates an amorphous structure. This combination therefore gives the resin more “bonding” opportunities for better adhesion.

As illustrated in [FIG. 3 ] or [FIG. 4 ], the casing 5 may house at least one elastic member 11 stressed in the longitudinal direction between a stop formed in the casing 5 and a longitudinal end of the microtube bundle, to ensure the longitudinal retention of the microtube bundle while allowing its expansion or contraction relative to the casing 5 in the longitudinal direction. Thus, a longitudinal end of the bundle of microtubes may be locked longitudinally while the other may be longitudinally free and retained by the elastic member 11. This makes it possible to absorb the contractions/expansions of the bundle of microtubes when the bundle is subjected to temperature variations, while preserving tightness.

As illustrated in [FIG. 3 ] or [FIG. 4 ], the elastic member may comprise, or consist of, a spring 11, notably a coil spring. Evidently, one or more stacked spring washers 12, notably of the Belleville type, may be used, as shown schematically in [FIG. 5 ].

The first fluid 2 (gas or liquid, for example, at a high pressure of between 5 bar and 100 bar) can enter via the inlet 6 (on the left in [FIG. 1 ]), penetrate into the microtubes 4, and pass out at the other end via the outlet 7. Simultaneously, the second fluid 3 (gas or liquid, for example, at a low pressure of between 1 bar and 99 bar) enters the casing 5 via an inlet (in the upper part of [FIG. 1 ]), circulates around the microtubes 4 and passes out via the outlet 9 (in the lower part of [FIG. 1 ]).

The embodiment of [FIG. 2 ] differs from that of [FIG. 1 ] only in that transverse deflectors 19 are provided in the casing 5 to force the second fluid to follow a serpentine path around the microtubes 4 between the inlet 8 and the outlet 9. This improves the heat exchange efficiency.

The casing 5 may consist of composite material, epoxy resin with glass fibers, polymer, metal, or any other appropriate material, notably the same material as that constituting the microtube 4. This minimizes the contraction differentials between the casing 5 and the microtubes 4.

The pressure differential between the pressure of the fluid circulating in the microtubes 4 (at high pressure, for example) and the pressure of the fluid circulating around the microtubes (at low pressure, for example) may be of the order of several bar or several tens of bar, of the order of a hundred bar for example.

As illustrated in [FIG. 6 ], the casing 5 may comprises an elastic area 14 lying along the longitudinal direction, such as a bellows, for absorbing variations of dimensions due to temperature changes.

Such a cryogenic heat exchanger 11 is particularly efficient, compact and light, by comparison with known cryogenic exchangers. Such an exchanger may be used, notably, as a cooling exchanger in a refrigeration and/or liquefaction device.

The heat exchanger 1 may, in particular, be used in a cooler/liquefier of the “Turbo Brayton” type.

[FIG. 7 ] illustrates an example of a cooling device 10. This comprises a working circuit 15 containing a working fluid (helium and/or hydrogen and/or argon and/or nitrogen and/or any other gas).

The working circuit 15 comprises at least a compressor 16 for the working gas, at least one heat exchanger 1 for cooling the compressed fluid, at least one expansion member 17 for expanding the working fluid, and at least one heat exchanger 117 for reheating the expanded working fluid. The expansion member may comprise, for example, at least one of a turbine, a Joule-Thomson valve, at least one aperture, etc.

For example, the at least one cooling heat exchanger 1 and/or the at least one reheating heat exchanger 1 may be a heat exchanger 1 of the aforesaid type with polymer microtubes.

For example, such a heat exchanger 1 may be used in such a device as a countercurrent heat exchanger for using the working fluid in heat exchange in two separate states of the cycle.

For example, at one end of the heat exchanger 1 (on the right in [FIG. 7 ]) the temperature of the fluid is reduced in the heat exchanger 1 (for example, from the non-cryogenic ambient temperature to a cryogenic temperature, notably between 130 K and 4 K), while at another end (on the left in [FIG. 7 ]) a flow of this fluid is reheated (for example, from a cryogenic temperature to a non-cryogenic temperature).

Evidently, the heat exchanger 1 is not limited to the above examples. Thus, for example, the heat exchanger may be configured for providing a heat exchange between more than two fluids (three, four or more). That is to say, separate portions of the microtubes 4 and/or of the space around the microtubes 4 may receive separate flows of fluids (different fluids or fluids of the same kind but at different or similar temperatures) for the purpose of exchanges of heat in the heat exchanger 1.

Such a heat exchanger 1 may, if necessary, provide heat exchange with another fluid (liquid nitrogen, for example).

Such a heat exchanger 1 may notably be used to provide pre-cooling of the fluid with a cold heat exchange fluid (liquid nitrogen, or any other fluid).

Such a heat exchanger 1 may also be used for cooling the working fluid at the outlet of a compressor. In this case, the working fluid may be made to exchange heat with a heat transfer fluid, such as water for example. In this case, the fluids involved in the heat exchange are not necessarily at cryogenic temperatures, and the exchanger could be replaced by a more conventional heat exchanger, but the aforesaid benefit of the heat exchanger 1 is still important.

Such a heat exchanger 1 may also be used for heating a cryogenic fluid contained in a storage system. The heat exchanger 1 is, for example, located outside the storage system and provides a heat exchange between the cryogenic fluid taken from the storage system and a hotter fluid (air, water or other heat transfer fluid) in order to vaporize it.

By way of example, such a heat exchanger 1 may be used for cooling and/or heating nitrogen, helium, hydrogen, argon or a mixture of some or all of these components in cryogenic form, by heat exchange with a fluid which may or may not be cryogenic, namely nitrogen, helium, hydrogen, argon or a mixture of some or all of the latter and/or water.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. A method for cryogenic cooling of a first fluid by heat exchange with at least one second fluid in a heat exchanger, the first fluid and/or the second fluid being at a temperature of between −100° C. and −273° C.; providing the heat exchanger that is of a type with polymer microtubes, that is to say comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 1 mm; and circulating one of the first and second fluids inside said microtubes while the other of the first and second fluids is circulated around said microtubes, wherein the heat exchanger comprises a casing in which the microtubes are arranged, the casing comprising a first inlet communicating with a first end of the microtubes, the casing comprising a first outlet communicating with a second end of the microtubes, the casing also comprising a second inlet and a second outlet communicating with the space around the microtubes.
 17. The method as claimed in claim 16, wherein the microtubes include material selected from the group consisting of: polyether ether ketone (PEEK), polytetrafluoroethylene (PTFE), polyetherimide, polyimides, polyamides, polycarbonates, any other plastic material compatible with use at said low temperatures, and combinations thereof.
 18. The method as claimed in claim 16, wherein the microtubes include a material comprising a mixture of polyetherimide (“Ultem”) and PEEK.
 19. The method as claimed in claim 16, wherein the microtubes include a material having a mass per unit volume of between 2700 kg/m³ and 900 kg/m³.
 20. The method as claimed in claim 16, wherein the microtubes have a diameter of between 0.1 mm and 5 mm.
 21. The method as claimed in claim 16, wherein a pressure differential between a pressure of the fluid circulated in the microtubes and a pressure of the fluid circulated around the microtubes is between 1 bar and 100 bar.
 22. The method as claimed in claim 16, wherein the microtubes are arranged in a bundle along a longitudinal direction in the casing.
 23. The method as claimed in claim 22, wherein at least one of the two longitudinal ends of the bundle of microtubes comprises a layer of rigid material such as a thermosetting material, that is configured to ensure the cohesion of the microtube bundle, the casing housing at least one elastic member stressed in the longitudinal direction between a stop formed in the casing and a longitudinal end of the microtube bundle, to ensure the longitudinal retention of the microtube bundle while allowing expansion or contraction relative to the casing in the longitudinal direction.
 24. The method as claimed in claim 16, wherein the microtubes are wound in a coil.
 25. The method as claimed in claim 16, wherein the microtubes are wound in a coil that is distributed around a central support core.
 26. The method as claimed in claim 25, wherein at least one of the two longitudinal ends of the bundle of microtubes comprises a layer of rigid material such as a thermosetting material, ensuring the cohesion of the microtube bundle, the casing housing at least one elastic member stressed in the longitudinal direction between a stop formed in the casing and a longitudinal end of the microtube bundle, to ensure the longitudinal retention of the microtube bundle while allowing expansion or contraction relative to the casing in the longitudinal direction.
 27. The method as claimed in claim 26, wherein the casing comprises an elastic area disposed along the longitudinal direction.
 28. The method as claimed in claim 26, wherein the elastic member comprises at least one of a spring, a coil spring, and one or more spring washers.
 29. The method as claimed in claim 28, wherein the casing comprises an elastic area disposed along the longitudinal direction.
 30. The method as claimed in claim 16, wherein separate portions of the microtubes and/or of the space around the microtubes receive separate flows of fluid(s) for the purpose of heat exchanges in the heat exchanger, such that an exchange of heat between more than two fluids occurs.
 31. The method as claimed in claim 16, wherein the cooling takes place in a process of cryogenic refrigeration and/or liquefaction of a fluid, the heat exchanger being located in a cryogenic refrigeration and/or liquefaction device.
 32. A device for cryogenic cooling of at least one first fluid by heat exchange with at least one second fluid, the device comprising: a heat exchanger providing an exchange of heat between the first fluid and the second fluid, the first and/or the second fluid being at a temperature between −100° C. and −273° C., the heat exchanger being of the type with polymer microtubes, that is to say comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 1 mm, and in that the heat exchanger comprises: a casing in which the microtubes are arranged, the casing comprising: a first inlet communicating with a first end of the microtubes, a first outlet communicating with a second end of microtubes, a second inlet and a second outlet communicating with a space around the microtubes, wherein the heat exchanger further comprises means for circulation of the first fluid inside said microtubes and means for circulation of the second fluid around said microtubes.
 33. The cooling device as claimed in claim 32, wherein the device comprises a working circuit containing a working fluid, the working circuit comprising at least one compressor for the working gas, at least one heat exchanger for cooling the compressed fluid, at least one expansion member for expanding the working fluid, and at least one heat exchanger for heating the expanded working fluid, wherein the at least one cooling heat exchanger and/or the at least one heating heat exchanger is of the type with polymer microtubes, that is to say comprising a plurality of microtubes made of polymer and having a diameter of between 0.1 mm and 10 mm, and comprising inlets and outlets for a first flow of working fluid and another fluid having a separate temperature from the temperature of the first flow of working fluid, to provide a heat exchange between the first flow of working fluid and the other fluid. 